Same problem for me as well.
Seems to switch output from HDMI to internal speaker as soon as there is a longer break, once even after starting and closing virtualbox.
17.3 did remember last used output, so must be something within 18.
Please Fix

If the last one, then it seems we're looking at a dumb Pulseaudio bug (if the second, then a perhaps not completely dumb Pulseaudio bug, and if the first perhaps even a not really all that dumb Pulseaudio bug).

Well, let us consider the above a potentially useful summary then. None of the described attempted solutions seem to have the potential to not work after a reboot yet work after (supposedly) a full shutdown, meaning something else (an upgrade?) may have rather fixed things -- but oh well, maybe still useful.

I had the problem for some time,i whas just going preferences and change to hdmi every time i startet my pc
Until i got so annoyed that i startet using my lm 17.3...hdd backup

I tried both login out, restart and shutdown after i had tried the commands in terminal
Listened to musik rest of the day and searching for various help on forum
Shutdown
woke up and checked my mail before rushing to work, tried as first to start winamp and play a number
and the sound came in my face at much too loud at 0500 am
checked the preferences, mm hdmi , no updating, upgrading or other serious inbetween

This started happening to me since I updgraded. 17 was fine. The audio keeps switching to the HDMI audio output source and I have to use "sudo pavucontrol" to revert the audio. This is really annoying!

Hi,
It is the same problem in Sarah 18.1.
Please get it fixed. It is very annoying.
Is it a help if I tell you that if I put it in "standby" it will wake up in HDMI as it should.
Could you please tell me to whom I could complain if the problem is not resolved soon.
To stop using Mint is a bad solution.
Yours sincerely
Amoko

Don't know if this will help others - but it solved this issue for me.

I added:
set-card-profile 1 output:hdmi-stereo+input:analog-stereo
to the end of /etc/pulse/default.pa

I figured out the card number (1) and output string from doing pacmd list-cards. For me the HDMI port shows up under index 1 and then the output string is under the profile section and looked sensible.

Running Mint 18 on a Lenovo Tiny M900 (awesome little unit). The fix has been "active" for almost a week now and works over multiple reboots/suspends/etc

I have NV 9600 M GT and it all worked on recent versions of Ubuntu. It still does on Win 7.
But I must manually mute the speakers (because they play alongside with TV) and switch to HDMI output on Serena.

Is there a way to switch automatically between speakers and HDMI just as I connect the cable?